Part 3: The Iodine Pit
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| Graph copied from accidont.ru (Author Viktor Dmitriev) |
During the first 25 minutes of April 26th, reactor power fell from 760 MW to 520 MW (either deliberately or due to SIUR Leonid Toptunov being overloaded following the shift change). TsTAI (ЦТАИ) shift supervisor Mikhail Elshin continues the narrative:
There
were a lot of people in the control room. The power reduction process
was under way. During that process there was a shift change. Shift No. 5
came on duty. Since the power reduction was ongoing, I was near the
left side of the operator's panel, behind Leonid Toptunov, and observed
the process along with everyone else standing nearby. While reducing
power the SIUR lost hold of the reactor and dropped it. The reactor
poisoned itself severely and the SIUR couldn't hold on.
From the perspective of Yuri Tregub:
When I glanced at the control panel to check the flow rates, I heard Akimov's voice: "Hold power" or "Maintain power!" ...something like that. I was standing near Toptunov. And I see that power is slowly falling... from what initial value, I don't know... But the guys told me that during the transition from LAR—that is the local automatic regulator—the SIUR didn't overcompensate enough on the main regulator, the reactor 'snapped down,' knocked out both automatics and power started dropping. All in all an unexpected thing, but it didn't worry me at all. Of course it wasn't great that the SIUR missed his mark, didn't switch over in time. But so what? That is all fixable. I was more worried by the coolant flow rate.
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| Leonid Toptunov |
At 0:28 power suddenly fell to a very low value generally reported as 30 MW. Neutron power was recorded as zero, although this may have been due to the value falling out of the lower bounds of the recording device. The initiating event for this drop in power was a transition (necessitated by the lower power level) between Local Automatic Control (LAR) and the system of Automatic Regulators (AR-1 and AR-2 automatic control rods). Toptunov was unable to eliminate an imbalance in the control system sensors fast enough, and the resulting unplanned insertion of automatic control rods stalled the reactor. This turn of events is described in virtually all sources as human error, although Dyatlov later defended Toptunov's handling of the situation and indicated that the Automatic Regulator had malfunctioned:
I do not blame Toptunov for the drop to 30 MW in the slightest. Any operator can have power drops when changing from one regulator to another. Sometimes bigger, sometimes smaller. Moreover, the regulator he switched to was faulty.
Controversy: Just how far did reactor power fall, and was the reactor shut down entirely or just barely hanging on? The answer allows commentators to either implicate or exonerate the shift personnel, and the answer is unclear. The Soviet prosecutor was anxious to prove that power had fallen to zero (the 30 MW would in this case be simply decay heat turning water to steam). On the flipside, current-day commentators often argue that power did not fall below even 100 MW, given that the turbine kept operating and certain signals were not recorded by the instruments. This theory is made more likely by the very low accuracy of the instruments as shown in the graph at the top of the page. Byzantine arguments rage to this day in the depths of the Russian-speaking internet among former reactor engineers and dilettantes both, meaning that the question is unlikely to ever be resolved.
Human Factors: At this point the operators had just minutes to make a decision, as xenon began smothering reactivity at an accelerated pace. If they interpreted the instruments as meaning that the reactor had shut itself down, then Section 6.1 of the regulations required pressing AZ-5 and waiting a day or more for the xenon to decay. There were exceptions to this rule provided ORM was high enough, but they were ambiguous in this particular situation. On the other hand the operators could interpret the situation as a 'partial power reduction' to a level above the 'minimal controllable level' (whose definition was also ambiguous). In such a case they were allowed to restore power. An operationally-led safety culture would militate for the more cautious, conservative interpretation of the situation and opt to shut down the reactor. However the operating culture of the time was very different, and the operators were likely to only regard the reactor as shut down when all control rods were fully inserted. The regulations gave them the latitude to continue and the scenario gave them plausible deniability. Furthermore, they would be unlikely to worry about the safety implications of xenon poisoning because they only planned to operate the reactor for another hour at most. Xenon would be increasing for the remainder of the shift, with no risk of it burning off when the reactor was operating at high power. Tregub's lack of concern here is instructive. It is likewise noteworthy that at Leningrad NPP (operated by a separate ministry that jealously guarded knowledge of the RBMK's flaws), restoring power was forbidden whenever it fell unexpectedly below 50%. They had already melted down part of their first reactor, and learned their lesson.
From the IAEA's INSAG-7 report:
The authors of this report believe that the drop in reactor power at 00:28 and subsequent power increase were largely to blame for the tragic consequences of the accident. The change in reactor operating conditions between 00:28 and 00:33 gave rise to a new xenon reshaping of the power density fields which the personnel were unable to control.
The second sentence refers to the fact that reactivity in the central portion of the core would become dampened, with neutron flux maximums at the very top and bottom of the core. The relatively high neutron flux at the bottom of the core would make the reactor highly vulnerable to the 'tip effect' of the descending controls rods.
In his book written just before his death, Anatoliy Dyatlov described the sequence of events as follows:
...I left the control room to examine several areas of interest before the shutdown. I always did that. In the first place, defects are more likely to reveal themselves during a transitional regime, and second of all at lower power one can more carefully examine rooms with higher radiation hazards.
I returned to the control room at 00:35. I can tell what time it was based on the reactor power diagram. From the door I saw leaning over the control panel not just operator L. Toptunov but also A. Akimov and the trainees B. Proskuryakov and A. Kudryavtsev. I don't remember who else. I approached and looked at the instruments. Reactor power was 50-70 MW. Akimov said that during the transfer from LAR to the regulators with side ionization chambers (AR) the power level fell to 30 MW. Now they were raising power. This did not excite or alarm me in the least. By no means was this anything out of the ordinary. I approved the power increase and stepped away from the control panel.
G.P. Metlenko and I discussed preparations for the 'Turbine Rundown Test' and made notes in the list of procedures. A. Akimov approached me and suggested raising power not to 700 MW(t), as written in the test program, but only to 200 MW(t). I agreed with him.
And while the operators were already raising power when I entered the control room, if I had been present during the drop, I would have allowed or ordered them to do so.
In response to similar testimony at the trial, the prosecution would call up bevy of witnesses to contradict Dyatlov's claim that he was absent during the drop in power.
Razim Davletbaev: Dyatlov was in the control room during the drop in reactor power.
Sergei Gazin: During the power reduction Akimov, Dyatlov and Tregub went up to Toptunov and did something there. Power fell almost to zero. Then they raised it to 200 MW(t).
Prosecutor's assistant: You confirm that Dyatlov was present at this time at the SIUR's panel?
Gennadii Metlenko: Yes, I think he was there... Something happened at 00:28... I wasn't near the control panel, but stepped away to the side. Everyone gathered near the panel. Then Dyatlov dabbed the sweat from his forehead with a handkerchief. Clearly he had been concerned about something.
Prosecutor: Dyatlov was in the control room the whole time?
Grigorii Lisyuk: He was absent at some point; I can't say how many times or how long.
In short, while one witness corroborated Dyatlov's brief absence from the control room, most testimony put him in the room during the drop in power or at least its immediate aftermath. No witnesses stated that Dyatlov gave the orders to restore reactor power. Dyatlov would later respond this these contradictions in a letter written from prison, to author Yuri Scherbak:
When 2-3 people stated that I was in the control room the whole time, I started to doubt myself: did my memory accurately recreate that visual image? After the nightmare of that night and that illness it is possible. But at the trial Metlenko said that I... wiped a bead of sweat from my brow. And that absolutely means that I had come from some room with higher temperatures... Why did Tregub and others have the impression that I never left the control room? I think that they all saw my return to the panel and saw me immediately after the drop in power. Then they formed the impression that I had been there the whole time... I want to say that regardless of whether I gave the order to raise power myself or Akimov made the decision, I still answer for it, since Akimov was subordinate to me.
My personal intuition is that Dyatlov believed his own words. His version of events was not implausible and did nothing to exonerate himself. At the same time the manic energy with which he writes does lead me to speculate that he was capable of latching onto false memories, following a period of intense trauma and persecution.
Courtroom drama aside, let us return to the moment of the power drop, when we interrupted an account by Yuri Tregub:
Who gave the command to raise power, this I don't know. But there was a command to raise power to 200 MW, and they raised power.
Akimov and I swapped places; I stood near the reactor power display and Akimov pulled the levers of the control regulators. And Toptunov started to raise control rods, to maintain power. For some reason he pulled them more from the third and fourth quadrants. I said to him: "What are you pulling them unevenly for? You need to pull them right there." And power fell. And from that moment I started to clue him in, which rods were free to be raised... I gave him advice. In some cases he agreed, in others not. I advised him: "Right there's a free rod and there's a free rod. You can remove it." He either took it or did it his way. I showed him these rods on the right side and up until the point where we reached 200 MW and turned on the automatic regulator, I didn't step away from him. We had to maintain power, prevent its fall.
TsTAI (ЦТАИ) shift supervisor Mikhail Elshin summarized the same process as follows:
Then Yuri Tregub began to bring the reactor up from zero using the manual control rods (the meter on the 'general power' device hovered around zero). At this time Lyonya Toptunov stood next to me. After Tregub stabilized power and turned on the automatic regulator, I left for my office. That was after 1:00, when I was sure that the automatic regulator was engaged.
Controversy: The disconnect between the two accounts above is clear. Tregub either advised Toptunov, or he actually replaced him at the controls for a short time. Elshin's version of events is clearly more popular in the Russian-speaking world, and I have even heard it repeated by a friend and colleague of Toptunov's. The suggestion is that Dyatlov drew on the more-experienced Tregub to do a job Toptunov could not (or in some version would not).
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| Photo credit: chidori@emptytriangle.com |
Myths/Misinformation: Most every reader will have been waiting for the part of the story where Toptunov (or in some versions Akimov) refuses to raise the power or otherwise violate safety procedures. Dire threats and verbal abuse from Dyatlov ensue. This striking example of 'information pollution' provides much of the drama to the most popular books, articles and documentaries about the accident. Even the official tour guides at the ChNPP tell a similar tale to visitors at Unit 3's control room. Most publications simply repeat an unsourced account in an infamously inaccurate and fictionalized book written by Grigori Medvedev.
Did Toptunov object or attempt to resist Dyatlov's orders to raise power on the stalled reactor? The sum total of eyewitness statements on this subject is as follows:
Sergei Gazin: Before the accident I did not hear any words spoken with raised voices, only instructions related to the execution of the test program. During the drop in power I approached the SIUR panel and saw what I understood to be intense efforts by Toptunov to increase and stabilize reactor power. I did not see anything resembling an attempt to replace Toptunov, nor any pressure exerted by you [Dyatlov] on Akimov and Toptunov, supposedly for refusing to raise power after the drop, nor any displeasure related to this drop. I believe that such a conflictive situation in the control room could not have passed unnoticed.
Yuri Tregub (in a letter to Dyatlov): Before the accident there were no discussions with raised voices among the operational personnel, as well as no displeasure voiced over the drop in power. There was also no attempt to replace L. Toptunov, and he carried out his duties during the entirety of the shift. After the drop in power the automatic power regulators were engaged and at shift supervisor A. Akimov's command—I assume in agreement with you and the station shift supervisor—power began increasing to 200 MW. I did not notice anything that could be interpreted as disagreement regarding the increase in power.
Yuri Tregub (to Yuri Scherbak): Dyatlov wielded the highest power at the block at that time. His authority and our trust... played a definite role. For us he was the highest authority. Untouchable authority. His word was law.
Prosecutor's assistant: Were all of Dyatlov's commands carried out without question?
Gennadii Metlenko: Yes, I think that it was like that.
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| The lever used to raise and lower control rods. |
Sources Cited:
- Nikolai Karpan, Revenge of the Peaceful Atom.
- Yuri Scherbak, Chernobyl.
- Razim Davletbaev, The Last Shift.
- Anatoliy Dyatlov, Chernobyl: How It Was.
- INSAG-7.
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